1. Field of the Invention
The present invention relates to an engine starter and electric generator system.
2. Description of the Relevant Art
Engines are usually associated with a starter motor which is energized by a battery as a power supply and an electric generator which charges the battery and supplies electric power to electric parts. The starter motor and the electric generator are costly to manufacture since each of their rotor and stator requires an expensive winding. Automotive engines are also associated with accessories such as an oil pump, a compressor, etc., as well as the starter motor and the electric generator, around an outer end of the crankshaft. Therefore, it is desirable to make compact the structure around the crankshafts of the automotive engines.
Japanese Laid-Open Patent Publication No. 63(1988)-202255 proposes a starter/generator which can operate selectively as a starter motor and an electric generator, so that the structure around the crankshaft of an engine is simplified and the cost of the engine is reduced. The disclosed starter/generator has a rotor directly coupled to the crankshaft and includes a housing which accommodates an armature coil connected to the driver circuit for the starter motor and a field coil connected to the rectifier circuit for the generator.
Generally, the ratio of the rotational speed of the rotor to the rotational speed of the crankshaft, as determined from the characteristics of a starter, is different from the ratio of the rotational speed of the rotor to the rotational speed of the crankshaft, as determined from the characteristics of an electric generator. With the starter/generator which is selectively operable as the starter motor and the generator, since the rotor is directly connected to the crankshaft and the ratio of the rotor speed to the crankshaft shaft remains constant, the characteristics of the starter/generator as both the starter and the generator cannot effectively be utilized fully.
An inverter circuit comprising power switching elements connected in a bridge form is known as an electric power supply for a starter motor. For example, Japanese Laid-Open Patent Publication No. 63(1988)-41667 discloses an inverter device composed of six power MOSFETs (metal-oxide semiconductor field-effect transistors) for driving a three-phase motor.
The disclosed inverter device includes a current-detecting resistor inserted in series with the power switching elements. When an overcurrent is detected on the basis of a voltage across the current-detecting resistor, gate driving voltages applied from a commutation logic circuit are cut off.
With the current-detecting resistor inserted in the path for supplying an electric current to the starter motor, however, the electric power supplied to starter motor is reduced by the electric power consumed by the inserted resistor, and hence the inverter device is not efficient enough. Since the gate driving voltage for the power switching elements is cut off when an overcurrent is detected, any failure caused by a short circuit of a certain power switching element can be detected only while the motor is in operation. If an FET connected to a positive power supply terminal and an FET connected to a negative power supply terminal are simultaneously shorted out, then any overcurrent cannot be cut off even when the gates are disabled. Therefore, the FETs will be excessively heated, a condition which is not desirable from the standpoint of safety, and also a wasteful consumption of electric power results.
A replaceable battery is used as the DC power supply for the inverter device. Should the battery be connected to the inverter device with the wrong polarities at the time of battery replacement or maintenance, the power switching elements may be damaged or degraded in characteristics.